In a wireless LAN system conforming to IEEE802.11 standard, access control employs a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) method. In its basic access procedure, that is, a decentralized management communication mode, each station detects the status of use of a wireless channel and determines the timing of sending data autonomously. In other words, each wireless station that wants to send data confirms the status of use of a wireless channel by performing carrier sense and postpones sending data so as to avoid a collision while sending from other wireless station is carried out.
However, stations that are out of their signal reachable areas cannot perform carrier sense, so that a collision occurs. Such a problem is known as a so-called “hidden terminal” problem. In order to avoid such a collision, it is known to use an RTS (Request To Send)/CTS (Clear To Send) method for reserving a channel by using a control message, that is, a handshake protocol. In this method, a sending station that wants to send data sends an RTS signal to a receiving station, and the receiving station replies a CTS signal to the sending station so as to permit the sending station to send data. Thus, since all adjacent stations of the sending station and the receiving station can be notified that the wireless channel is used, the collision frequency can be reduced.
On the other hand, in the CSMA/CA method, when an adjacent station performs carrier sense and it is judged that a wireless channel is used, the adjacent station cannot carry out sending. This is known as a so-called “exposure terminal (exposed terminal) problem.”
FIG. 15 is a plan layout view showing wireless communication devices (wireless terminals) in a conventional wireless network system. This system includes A terminal 301, B terminal 302, C terminal 303 and D terminal 304. It also includes signal reachable area 311 of B terminal 302 and signal reachable area 312 of A terminal 301. While A terminal 301 sends data to B terminal 302, since C terminal 303 performs carrier sense of a signal sent by A terminal 301, sending to D terminal 304 is suppressed. However, actually, even if C terminal 303 sends a signal to D terminal 304, since the signal sent by C terminal 303 does not reach B terminal 302, the signal can be sent without occurring a collision. In this way, an adjacent terminal, which is in a position where a collision with the receiving terminal does not occur, is deprived of an opportunity of sending. Thus, the wireless transmission efficiency is deteriorated. This problem is called an exposure terminal problem.
As a technology for solving this exposure terminal problem, a DBTMA (Dual Busy Tone Multiple Access) method is known. An example of this method is disclosed in Zygmunt J. Haas et. al, “Dual Busy Tone Multiple Access (DBTMA)—A Multiple Access Control Scheme for Ad Hoc Networks”, IEEE TRANSACTIONS ON COMMUNICATIONS, 2002, VOL. 50, NO. 6, p. 975-985. The DBTMA method uses two out-of-band busy tones in order to notify the adjacent stations that a wireless channel is used. One of the busy tones shows a send busy state another shows a receive busy state. The frequencies of these two busy tones are largely apart from each other in order to avoid interference.
FIG. 16 is a timing chart showing an operation of each radio communication device (wireless communication device) in a conventional wireless network system using the DBTMA method. Provided that each terminal of this system is in the same position as each terminal of the system shown in FIG. 15, an operation thereof is described with reference to FIGS. 15 and 16 hereinafter.
A terminal 301 sends RTS signal 501 and send busy signal 1601 at the same timing. B terminal 302 receives RTS signal 501 and sends receive busy signal 1602 when B terminal 302 is ready to receive data. When A terminal 301 receives receive busy signal 1602 and judges that B terminal 302 is ready to receive data, A terminal 301 sends only data 503 without sending send busy signal 1601. B terminal 302 continues to send receive busy signal 1602 until receiving of data 503 is completed.
With such an operation, since a terminal in signal reachable area 311 of B terminal 302 refrains from sending data, a collision can be avoided. A terminal such as C terminal 303 that is in signal reachable area 312 of A terminal 301 and out of signal reachable area 311 of B terminal 302 detects neither receive busy signal 1602 of B terminal 302 nor send busy signal 1601 of A terminal 301. Therefore, such a terminal can start to send data to D terminal 304, so that an exposed terminal problem can be avoided.
In a conventional technology, however, it is necessary to provide a band for busy tones out of the band of a signal and the layout of frequency is limited. As a result, the configuration of a sending and receiving device becomes complicated. Furthermore, depending upon the state of a propagation path, a busy tone may disappear due to a multipath wave, so that a station may mistake that sending can be carried out and sends a signal. Such a signal collides with a signal received by an adjacent station, resulting in deteriorating the transmission efficiency. Furthermore, the conventional technology assumes that a collision does not occur and does not consider the replay of an Ack (Acknowledgement) signal. Therefore, in a harsh wireless environment in which fading and the like occurs, an effective automatic retransmission cannot be applied, thus deteriorating the transmission efficiency.